Hydroxyphenylalkyleneyl isocyanurates such as (3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate are useful antioxidants for organic materials subject to attack and degradation by heat and oxygen. They find particular utility in polyolefins. It is desirable to improve the antioxidant and other properties of these materials, particularly at high temperatures. It has been observed that certain phosphorus containing compounds, U.S. Pat. No. 3,351,483, are useful in enhancing the ultraviolet resistant properties of the hydroxyphenylalkyleneyl isocyanurates. Although many phosphorus compounds are available, only a few have been found to have value as enhancers for the stabilizing activity of hydroxyphenylalkyleneyl isocyanurates, and the best of these are deficient in several properties, particularly hydrolytic instability, both in storage and in use. Improved combinations of hydroxyphenylalkyleneyl isocyanurates and phosphorus containing compounds are desired that result in synergistic combinations as to antioxidant properties, especially as shown by oven aging, that are not subject to hydrolysis, and that provide excellent color and processing properties to materials containing the combination of hydroxyphenylalkyleneyl isocyanurate and phosphorus compound.